The present invention relates to a process for preparing a fluoropolymer and a solution catalyst capable of providing a fluoropolymer. More particularly, the invention relates to a process for preparing a perfluoropolyether, comprising polymerizing hexafluoropropylene oxide in a special polymerization initiator-containing solution.
Bifunctional perfluoropolyether oligomers can undergo crosslinking reaction or can be made to have high molecular weight by taking advantage of functional groups present at both ends, and therefore they are useful as resins for sealants, adhesives and vulcanized rubber molded articles having excellent solvent resistance and chemical resistance. The bifunctional perfluoropolyethers can be favorably used as resins which are used for, for example, molded articles, such as O-ring, gasket, diaphragm and tube, sealing agents of chemical plant pipes, sealing agents of tank flanges, and adhesives.
It has been heretofore known that the bifunctional perfluoropolyether (sometimes referred to as xe2x80x9cPFExe2x80x9d hereinafter) is prepared by polymerizing hexafluoropropylene oxide (sometimes referred to as xe2x80x9cHFPOxe2x80x9d hereinafter) in the presence of an appropriate catalyst or polymerization initiator.
In the polymerization of HFPO, chain transfer caused by compounds other than the polymerization initiator, e.g., chemical species such as H2O, HF and a carbonyl group, is very liable to take place, so that it is important to eliminate inclusion of the chemical species other than the polymerization initiator to the utmost thereby to inhibit polymerization initiation from such chemical species and to conduct polymerization reaction under the conditions where formation of by-products such as compounds other than the intended bifunctional perfluoropolyether can be inhibited. In particular, H2O is present anywhere, e.g., in air, and there is a high possibility of inclusion of H2O into the polymerization system. Hence, it is very important to eliminate inclusion of H2O.
It is known that in order to enhance selectivity of the bifunctional perfluoropolyether, the polymerization reaction needs to be promoted with maintaining the polymerization temperature of HFPO as low as possible. It is also known that lowering of the polymerization temperature in the polymerization of HFPO not only inhibits chain transfer but also enhances selectivity of the reaction and degree of polymerization of the resulting polymer. Further, it is also known that use of the bifunctional initiator as the polymerization initiator reduces formation of a monofunctional polymer (by-product).
In xe2x80x9cJ. MACROMOL. SCI. xe2x80x94CHEM.xe2x80x9d, 48(3), 499-520 (1974), there is described a process wherein hexafluoropropene (sometimes referred to as xe2x80x9cHFPxe2x80x9d hereinafter) is allowed to be present during the polymerization of HFPO to inhibit chain transfer and to increase degree of polymerization of the resulting polymer.
In U.S. Pat. No. 3,660,315 and Japanese Patent Publication No. 5360/1978, there is described a process for preparing a bifunctional perfluoropolyether, comprising mixing cesium fluoride, tetraglyme and FOCCF(CF3)OCF2CF2OCF(CF3)COF with one another and allowing HFPO to react, at a low temperature, with a compound of the following formula contained in the resulting homogenous solution from which extra cesium fluoride has been separated. 
The present inventors have made an attempt to synthesize a compound represented by the above formula and to prepare a bifunctional perfluoropolyether from HFPO in the presence of the resulting compound, in accordance with the process described in U.S. Pat. No. 3,660,315 and Japanese Patent Publication No. 5360/1978. As shown in Comparative Example 1 or 2 in this specification, however, when the degree of aging of the catalyst or the concentration of the catalyst is out of a certain range, the degree of polymerization and the selectivity of the bifunctional perfluoropolyether are lowered, resulting in lack of utility.
In Japanese Patent Laid-Open Publication No. 101788/1998, there is described a process for preparing a bifunctional polyether, wherein in order to increase selectivity of the reaction, heat of polymerization is removed from the highly viscous HFPO polymerization system to lower the polymerization temperature, and in order to inhibit chain transfer, a liquefied gas of fluorocarbon of 1 to 4 carbon atoms is added to the reaction system and HFPO is polymerized with evaporating the liquefied gas from the polymerization system. In this process, however, absence of extra HFPO is necessary for inhibition of side reaction such as formation of a monofunctional product, and it is sometimes difficult to control the reaction.
It is difficult to completely remove a chain transfer-causing compound, such as H2O, HF or a carbonyl group-containing compound, from HFPO that is a starting material of a perfluoropolyether. Accordingly, there has been desired development of a process for preparing a bifunctional perfluoropolyether wherein even if such a chain transfer-causing compound is present in a trace amount, a bifunctional perfluoropolyether can be readily obtained with high degree of polymerization and high selectivity.
Under such circumstances, the present inventors have earnestly studied in order to solve the above problems. As a result, they have found that when a polymerization initiator-containing solution having been subjected to a certain treatment is used as a polymerization catalyst, a bifunctional perfluoropolyether can be obtained with high degree of polymerization and high selectivity. The present inventors have also found that this polymerization initiator-containing solution is in a liquid state even at low temperatures. Based on the finding, the first invention has been accomplished.
Moreover, the present inventors have found that when a polymerization initiator-containing solution which contains no free Cs+Fxe2x88x92 is used as a polymerization catalyst, a bifunctional perfluoropolyether can be obtained with high degree of polymerization and high selectivity. The present inventors have also found that this polymerization initiator-containing solution is in a liquid state even at low temperatures. Based on the finding, the second invention has been accomplished.
It is an object of the present invention to provide a convenient process for preparing a bifunctional perfluoropolyether with high degree of polymerization and high selectivity. In particular, it is an object of the first invention to provide a solution catalyst which serves as a polymerization catalyst, can be in a liquid state even at a low temperature during the polymerization reaction, can be homogeneously dispersed in the polymerization reaction system and exhibits low chain transfer property and high activity.
The summary of the present invention is described below.
The polymerization initiator-containing solution according to the invention is a polymerization initiator-containing solution capable of being obtained by mixing a perfluorodicarboxylic acid fluoride (A) represented by the following formula (I) with CsF in an aprotic polar solvent with stirring to conduct reaction and thereby form a polymerization initiator (B) and then allowing the reaction solution to stand for not less than 72 hours at a temperature of 0 to 30xc2x0 C.;
FOCxe2x80x94Rfxe2x80x94COFxe2x80x83xe2x80x83(I) 
wherein Rf is a perfluoroalkylene group having 1 to 4 carbon atoms or a perfluoroalkylene group having 2 to 10 carbon atoms and an ether bond.
The process for preparing a polymerization initiator-containing solution according to the invention comprises mixing a perfluorodicarboxylic acid fluoride (A) represented by the following formula (I) with CsF in an aprotic polar solvent with stirring to conduct reaction and thereby form a polymerization initiator (B) and then allowing the reaction solution to stand for not less than 72 hours at a temperature of 0 to 30xc2x0 C.;
FOCxe2x80x94Rfxe2x80x94COFxe2x80x83xe2x80x83(I) 
wherein Rf is a perfluoroalkylene group having 1 to 4 carbon atoms or a perfluoroalkylene group having 2 to 10 carbon atoms and an ether bond.
The process for preparing a perfluoropolyether according to the invention comprises polymerizing hexafluoropropylene oxide in the presence of the above-mentioned polymerization initiator-containing solution.
The polymerization initiator (B) contained in the polymerization initiator-containing solution is preferably a compound represented by the following formula (II):
CsOCF2xe2x80x94Rfxe2x80x94CF2OCsxe2x80x83xe2x80x83(II) 
wherein Rf is a perfluoroalkylene group having 1 to 4 carbon atoms or a perfluoroalkylene group having 2 to 10 carbon atoms and an ether bond.
The concentration of the polymerization initiator (B) in the polymerization initiator-containing solution is preferably not less than 4xc3x9710xe2x88x924 mol/g.
The polymerization of the hexafluoropropylene oxide is preferably carried out at a temperature of not higher than xe2x88x9230xc2x0 C.
In the polymerization of the hexafluoropropylene oxide, it is preferable to further use hexafluoropropylene in combination in an amount of 20 to 50% by weight based on the amount of the hexafluoropropylene oxide.
The perfluorodicarboxylic acid fluoride (A) is preferably represented by the following formula (III): 
wherein Rf1 is a perfluoroalkylene group having 2 to 4 carbon atoms.
The aprotic polar solvent is preferably diglyme, triglyme, tetraglyme or sulfolane.
The process for preparing a perfluoropolyether according to the invention comprises polymerizing hexafluoropropylene oxide in the presence of a polymerization initiator-containing solution which comprises a polymerization initiator (Bxe2x80x2) represented by the following formula (IV) and an aprotic polar solvent;
(FOC)xxe2x80x94Rf2xe2x80x94(CF2OCs)yxe2x80x83xe2x80x83(IV) 
wherein Rf2 is perfluoroalkylene having 1 to 4 carbon atoms or perfluoroalkylene having 2 to 10 carbon atoms and an ether bond, and x and y are numbers satisfying the conditions of x+y=2 and 0.1 less than y less than 2.
Rf2 in the formula (IV) is preferably represented by the following formula (V): 
wherein Rf3 is perfluoroalkylene having 2 to 6 carbon atoms.
The polymerization of the hexafluoropropylene oxide is preferably carried out at a temperature of not higher than xe2x88x9230xc2x0 C.
The concentration of the polymerization initiator (Bxe2x80x2) in the polymerization initiator-containing solution is preferably not less than 4xc3x9710xe2x88x924 mol/g.
In the polymerization of the hexafluoropropylene oxide, hexafluoropropylene can be further used in combination in an amount of 20 to 50% by weight based on the amount of the hexafluoropropylene oxide.
The polymerization initiator (Bxe2x80x2) is preferably a compound obtained by allowing a perfluorodicarboxylic acid fluoride (Axe2x80x2) represented by the following formula (VI) to react with CsF in an aprotic polar solvent;
FOCxe2x80x94Rf4xe2x80x94COFxe2x80x83xe2x80x83(VI) 
wherein Rf4 is a perfluoroalkylene group having 1 to 4 carbon atoms or a perfluoroalkylene group having 2 to 10 carbon atoms and an ether bond.
The perfluorodicarboxylic acid fluoride (Axe2x80x2) is preferably represented by the following formula (VII): 
wherein Rf5 is a perfluoroalkylene group having 2 to 4 carbon atoms.
The molar ratio (CsF/perfluorodicarboxylic acid fluoride (Axe2x80x2)) of the CsF to the perfluorodicarboxylic acid fluoride (Axe2x80x2) used is preferably not less than 0.1 and less than 2.
The aprotic polar solvent is preferably diglyme, triglyme, tetraglyme or sulfolane.